A typical belt tensioner includes a fixed structure, a pivoted structure mounted on the fixed structure by a pivot bearing for pivotal movement about a pivotal axis and a belt tensioning pulley mounted on the pivoted structure by a rotational bearing for rotational movement about a rotational axis parallel with the pivotal axis. A torsion spring is provided for resiliently biasing the pivoted structure to pivot from a first limiting position corresponding to a loosened belt mounting position toward a second position during which the belt is engaged and tensioned. The torsion spring applies a torque to the pivoted structure which when the belt is tensioned is equal and opposite to a belt load torque. The spring torque decreases as the pivoted structure moves from its first position toward its second position and so does the belt load torque. While the belt load torque decreases over the range of movement provided, it is desirable that the belt load force and hence the belt tension be maintained relatively constant throughout the range of movement provided. The pulley is positioned with respect to the belt such that the lever arm through which the belt load force acts decreases as the pivoted structure moves from the first position to the second position so as to maintain the static belt load force substantially constant. In this way, the belt tensioner operates to maintain a constant static belt tension throughout a range of movement capable of accomodating belt wear and extension.
When the belt is moved, the dynamic belt load force acting on the tensioning pulley varies from the static load force resulting in movements away from the static equilibrium position which are either resisted by or effected by the torque applied by the spring. If this resilient movement is allowed to take place without damping control, a resonant vibratory movement of the pulley can ensue with disastrous results.
In commonly assigned copending application, Ser. No. 090,495, filed Aug. 28, 1987, there is disclosed a releasable belt load damping arrangement which minimizes the pivotal movements requiring damping. The arrangement provides a damping torque resistance to pivotal movements which is established by virtue of the engagement of a bearing surface of the sleeve pivot bearing and a bearing surface of either the fixed structure or pivoted structure slidable with respect thereto. The surfaces are pressed together by the belt load force of the pivoted member and a reactionary force of the fixed structure. The magnitude of the damping torque resistance is a function of the magnitude of the belt load and reactionary forces, the coefficient of friction of the materials defining the surfaces and the radial distance from the pivotal axis to the surfaces. In order to establish a damping torque resistance of sufficient magnitude, the last variable is, in accordance with the principles enunciated in the application, increased significantly, preferably to the point that the sleeve bearing is of a size to encircle the rotational bearing. This relationship enabled the two bearings to be brought into axial coextensive relation with the belt engaging exterior periphery or rim portion of the pulley so as to insure that the belt load force is transmitted to the pivot bearing in an axially balanced fashion thus achieving uniform wearing characteristics and hence longer bearing life.
As previously indicated, the nature of the counterbalancing spring torque and belt load torque is such that there is a limit to the angular extent of pivotal movement which the pivoted structure can have and still maintain a generally constant belt load force throughout the range of pulley movement provided. This range of angular movement is approximately 70.degree.. Consequently, the amount of pulley movement provided is a direct function of the distance between the pulley rotational axis and the pivotal axis of the pivoted structure. In the axially aligned and balanced arrangement such as disclosed in the aforesaid application, this distance must necessarily be less than the radius of the pulley; consequently, as pulley movement range is increased, pulley size must likewise increase. There are other known types of axially aligned and balanced belt tensioners which suffer from the same limitations. See, for example, U.S. Pat. Nos. 4,077,272, 4,504,252, 4,551,120, and 4,634,407. Because of these limitations, there is a need to provide for a greater range of pulley movement without sacrificing the ability to maintain a generally constant static belt load force throughout the range of movement provided.